The invention relates to a micro-optic viewing arrangement having a multiplicity of microfocusing elements that focus in a target focal-plane and form a viewing grid. The invention also relates to micro-optic representation arrangements that have a micro-optic viewing arrangement according to the invention, security elements for securing the authenticity of data carriers that have a micro-optic viewing arrangement according to the invention, and data carriers that have a micro-optic viewing device according to the invention. The invention furthermore relates to a method for manufacturing the micro-optic viewing arrangement and a method for manufacturing data carriers having the micro-optic viewing arrangement according to the invention.
Data carriers, such as value documents or identification documents, bank notes, deeds, checks, but also other objects of value, such as branded articles, are often provided for safeguarding purposes with security elements which permit a check of the authenticity of the data carrier and which at the same time serve as protection from unauthorized reproduction. The security elements can be configured for example in the form of a security thread embedded in a bank note, a window security thread, a cover foil for a bank note with a hole, an applied strip or a self-supporting transfer element, such as a label, which is applied after its manufacturing to the value document. For the purposes of the present invention, by a value document there are to be understood also security papers, security papers constituting a precursor to a value document not yet fit for circulation.
A special role has been played here for some time by security elements having optically variable elements which convey different pictorial impressions to the viewer from different viewing angles, because optically variable elements cannot be reproduced even with high-quality color copiers. For this purpose, the security elements can be equipped with security features in the form of optically-diffractively active microstructures or nanostructures, for example with embossed holograms or other hologram-like diffraction structures. However, such structures are more and more used also for purposes other than security applications, which is why their value as security features has gone down, because they are often rather perceived as a design feature and receive little attention.
It has also been customary for some time to use lens systems as security features. For example in the print EP 0 238 043 A2 there is described a security thread made of a transparent material having embossed on its surface a grid comprising a plurality of parallel cylindrical lenses. The thickness of the security thread is chosen such that it corresponds approximately to the focal length of the cylindrical lenses. A printed picture is applied in exact register to the opposite surface, the printed picture being designed with consideration of the optical properties of the cylindrical lenses.
A more striking variant are so-called moiré magnification arrangements. Such a moiré magnification arrangement is disclosed in the print WO 2006/087138 A1. The security element disclosed therein has at least a first and a second authenticity feature. The first authenticity feature here comprises a first arrangement with a multiplicity of focusing elements which are present in a first grid, as well as a second arrangement with a multiplicity of microscopic structures which are present in a second grid. The first and the second arrangement are arranged relative to each other such that the microscopic structures of the second arrangement are to be seen magnified upon viewing through the focusing elements of the first arrangement.
The basic mode of function of moiré magnification arrangements is described in the article “The moiré magnifier”, M. C. Hutley, R. Hunt, R. F. Stevens and P. Savander, Pure Appl. Opt. 3 (1994), pp. 133 to 142. Very briefly, moiré magnification designates the phenomenon which occurs upon viewing a grid comprising identical picture objects through a lens grid with approximately the same grid dimension. As with every pair of similar grids, there results here a moiré pattern consisting of a periodic arrangement of magnified and, where applicable, rotated images of the elements of the picture grid.
Further micro-optic magnification arrangements of the moiré type are described in the applications DE 10 2007 029 203 A1 and WO 2009/000529 A2 described, and so-called modulo magnification arrangements are described in the applications WO 2009/000527 A1 and WO 2009/000528 A1. All these micro-optic magnification arrangements contain a motif picture with microstructures which reconstructs a specified target image upon viewing with a suitably coordinated viewing grid. As explained more precisely in the above-mentioned prints and applications, there can thereby be produced a multiplicity of visually attractive magnification effects and movement effects which lead to a high recognition value and a high forgery resistance of the security elements or value documents equipped therewith.
In the known moiré magnification arrangements (moiré magnifiers) the picture to be represented consists of individual motifs which are periodically arranged in a grating. The motif picture to be viewed through the lenses represents a version of the image to be represented, which version is greatly reduced in size, the area assigned to each single motif maximally corresponding approximately to a lens cell. Due to the smallness of the lens cells, only relatively simple structures come into consideration as single motifs. In contrast thereto, in “modulo mapping” the target image is in general a single image, i.e. it need not necessarily be composed of a grating of periodically repeated single motifs. The target image can represent a complex single image with high resolution. In modulo magnification arrangements, there is not necessarily involved a moiré effect, and it need not necessarily be a magnification. Rather, the term “mapping” refers to arbitrary imagings.
It is often desirable to equip micro-optic representation arrangements on their upper side, i.e. the lens side, with coatings which fulfil a given function, for example with dirt-repellent coatings and/or with adhesive layers.
In particular heavily used value documents such as bank notes are often equipped with dirt-repellent coatings so as to prolong their period of circulation. If a bank note or any other value document has a micro-optic representation arrangement, for example a moiré magnifier or a modulo mapping arrangement, it is, of course, desirable to coat also this arrangement, because it would otherwise become more soiled than the rest of the bank note.
However, micro-optic representation arrangements only provide a sharp imaging, when the microstructures to be imaged are located exactly in the focal plane of the lenses or of the any other microfocusing elements. An over-coating of the lenses, for example with a dirt-repellent coating, leads to a deterioration and in the worst case to a destruction of the optical effect, because the focal distance (focal length) of the microfocusing elements changes through the over-coating, and thus the microstructures no longer lie exactly in the focal plane of the microfocusing elements: the imaging becomes unsharp or can even completely “disappear”.
A similar problem occurs upon embedding window security threads having micro-optic representation arrangements. The window security threads have on their upper side, the lens side, regions that are not covered by paper (window), and regions that are covered by paper. In the window regions a dirt-repellent coating should be advantageously provided, while in the covered regions an adhesive layer should be provided, in order to bond the thread to the paper regions resting thereon. Without bonding the paper bars lying above the thread may easily lift from the thread and can thus be quickly damaged.
It is basically possible to perform a coating of the focusing elements with adhesive in the regions of a security thread that are covered by paper bars, because the thread is not visible in these regions and a deterioration of the optical effect is therefore irrelevant. In practice, however, it is hardly possible to selectively coat with adhesive only those regions of a security threads that are later hidden under paper bars. Furthermore, it is hardly possible in practice to selectively leave out given partial regions, such as the window regions of a window security thread, when a bank note is finally equipped with a dirt-repellent coating. Therefore, one has to decide whether one does without a bonding of the window security thread to the paper bars lying above the thread and/or without equipping it with a dirt-repellent coating, or whether one accepts a deterioration of the optical quality of the thread by over-coating the lenses.